In the movie "Hedwig and the Angry Inch," there's a song called "Origin of Love." It describes how soulmates were originally one person until the gods separated them to punish mankind, and we go around trying to find our other halves. It's specifically talking about romantic love, but there is one part of the song that I think also accurately describes childbirth or a mother/child relationship:
Last time I saw you
We had just split in two.
You were looking at me.
I was looking at you.
You had a way so familiar,
But I could not recognize,
Cause you had blood on your face;
I had blood in my eyes.
But I could swear by your expression
That the pain down in your soul
Was the same as the one down in mine.
That's the pain,
Cuts a straight line
Down through the heart;
We called it love.
I'm thinking of getting those last four lines as a tattoo. However, as I mentioned, the song is REALLY describing romantic love (and the very next line makes a reference to sex). Would it be weird for me to get lines from that song for my son?